This invention relates to a microwave integrated circuit which includes signal line patterns that are formed on a ceramic or an other dielectric substrate for creating a device such as a directional coupler.
FIG. 1 shows a directional coupler using microstrip line patterns which appears as FIG. 2-18 and FIG. 2-23(b) in Tsushin-Yo Maikuroha Kairo (Microwave Circuits for Communications) published by the Institute of Electronics and Communications Engineers of Japan. This circuit includes a dielectric substrate 1 made of a ceramic or a similar material, microstrip lines 4, 5, and 6 formed on the dielectric substrate 1, and a ground plane 7. The coupling is formed by the microstrip lines 5 and 6. The microstrip lines 4 are transmission lines that connect the microstrip lines 5 and 6 to input/output edge ports 4-1 to 4-4.
FIG. 2 shows another microstrip coupler configuration which appears as FIG. 2-25 in the above mentioned publication as an example of a 3dB directional coupler. The coupled microstrip lines 5 and 6 have an interdigital configuration and are connected by wires 8. One reason for this configuration is to attain a tighter coupling.
These prior-art directional couplers operate as follows. In the example of FIG. 1, as the microwave power input to the edge port 4-1 of the microwave transmission path formed by the ground plane 7 and the microstrip line 4 traverses the coupled strip line 5, part of the power is transferred to the coupled strip line 6 and is transmitted to the edge port 4-2. Most of the remaining power which is not transferred reaches the edge port 4-3. Any desired coupling ratio, hence any desired power output at the edge port 4-2, can be achieved by an appropriate selection of the gap G between the coupled strip lines 5 and 6, the thickness H of the substrate 1, and the width W of the coupled strip lines 5 and 6.
The example of FIG. 2 differs from the example of FIG. 1 only in the disposition of the edge ports numbered 4-1 to 4-4 with the circuit operating similarly. The coupling between the ports 4-1 and 4-2 is 3dB, so 3dB power is also transmitted to the edge-port 4-3.
Next, a bandpass filter will be shown in a further example of the prior art. FIG. 3 illustrates a microstrip coupler configuration, which appears as FIG. 2-85(b) in the above-mentioned publication, as an example of a half-wavelength side-to-side coupling filter. The circuit includes an input strip line 11, coupled strip lines 12 to 16, and an output strip line 17. In this bandpass filter, the microwave power input at the edge port 11-1 is supplied from the input strip line 11 to the coupled strip line 12, and is propagated through the series of coupled strip lines 12 to 16 and the output strip line 17 to the output edge port 17-1. The signal thus transmitted from the input port 11-1 to the output port 17-1 contains substantially only those frequency components in the passband of the filter and the frequency components outside the passband are reflected.
A problem with the prior-art directional coupler configurations of FIG. 1 and FIG. 2 is that when tight coupling is required, the gaps between the coupled strip lines must be extremely narrow. In consequence, the design values of the coupling can be attained only by extremely accurate patterning, and the difficulty in achieving extreme accuracy impairs the productivity of the fabrication process.
Another consequence of the narrow gaps between the coupled strip lines is an inability of the circuits to withstand high levels of applied power. A further problem is that the planar arrangement of the coupled strip lines requires the dielectric substrate 1 to have a large surface area.
The preceding problems are also present in devices of FIG. 3, such as filters that use coupled microstrip lines.